How much would you think of? Actually getting bumped by a horse is deadly enough irl I could imagine. However as this is a game, it should be kinda low compared to a hit by arrows/bolts/swords/other stuff.
But how much? 5%? Sounds a bit low, considering getting bumped by a horse at full speed or a plated one. 10%? Not sure about that either.
I actually prefer it as now. If you got light armor, you should get more ath to jump out of the way or a long piercing weapon to stop the horse... I usually ignore bump dmg of horses though, whenever I'm around with 18 str+, some IF and medium armor, I never mind the low dmg a horse causes to me when being bumped (except it was a plated charger for example
).
Edit: After all, the bump should cause someone to be stunned for some time by falling to the ground. It's rather meant as a help for the infantry, than as real damage to kill someone.